Claiming this is semi-decentralized is confusing, and seemingly wanting to borrow from the recent success of decentralized systems (like IPFS, ours, GUN, and others) without being honest: There system is distributed, not decentralized. In the same way "Serverless" totally requires using servers.
Otherwise, very good article.
1- Why C-instance can come from any node without a paxos phase 1a Prepare message, is it because each node (R0 to R4) have its own distinct replication log for C-instance?
2-When sequencer receive a C-Accept why is it safe to assume this value was successfully accepted by other replica without receiving a paxos phase3 Commit?
3-If replicating large value, are the value only sent in C-instance message and not in O-instance messages?
1& 2) In fact the C-instance messages do not conflict with each other and gets accepted immediately. These messages do not even need a ballotnum, but the ballotnum used is that of the O-instance to denote sort of an epoch of which sequencer the sender thinks is still in-charge.
3) If replication messages are large, you can just order the "commands" referring/pointing to them via Paxos, and not necessarily the data itself.
Paxos has different performance characteristics, different implementations, and more maturity.
There is no simple answer to your question unless you make it more specific.
I am no expert on Paxos - just hoping for an explanation.
Where it get complicated is trying to build a practical replicated log system using Paxos. Raft just happen to clearly and completely define this use case.
What SDPaxos or EPaxos try to achieve is good performance over WAN. Something that Raft and any Paxos variant that rely on a stable leader are very bad at.
This can’t be easily added to raft because the main reason the raft algorithm is simpler is because it assume a stable leader in every operation except leader election.